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Hypocapnea, sometimes incorrectly called acapnea, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation.

Even when severe, hypocapnea is normally well tolerated. However, hypocapnea causes cerebral vasoconstriction, leading to cerebral hypoxia and this can cause transient dizziness, visual disturbances, and anxiety.

A low partial pressure of carbon dioxide in the blood also causes alkalosis (because CO2 is acidic in solution), leading to lowered plasma calcium ions and nerve and muscle excitability.

This explains the other common symptoms of hyperventilation — pins and needles, muscle cramps and tetany in the extremities, especially hands and feet.

Hypocapnea is sometimes induced in the treatment of the medical emergencies, such as intracranial hypertension and hyperkalaemia.

Because the brain stem monitors the level of blood CO2 in blood to regulate breathing, hypocapnea can suppress breathing to the point of blackout from cerebral hypoxia. Self-induced hypocapnea through hyperventilation is the basis for the deadly schoolyard choking game.

Deliberate hyperventilation has been unwisely used by underwater breath-hold divers to extend dive time but at the risk of shallow water blackout, which is a significant cause of drowning.